Since when was 20% of £250, sixty two pounds? If the extra £12 is for the box of coving thats cheap coving. I'm with Joe, i though £250 was expensive but a total of £312 is very expensive.
The firm do sound slack, good advice to contact trading standards but i really do think you need to have a face to face with the guy/firm and you sound like a good customer and a nice lady thats made a small mistake in not reading the quote properly, if the guy has any sense he'd make you a good offer of a compromise in the hope of getting more work, however at those prices i'd expect his work to be absolute top draw and have a finer eye for detail in the paperwork.
The firm do sound slack, good advice to contact trading standards but i really do think you need to have a face to face with the guy/firm and you sound like a good customer and a nice lady thats made a small mistake in not reading the quote properly, if the guy has any sense he'd make you a good offer of a compromise in the hope of getting more work, however at those prices i'd expect his work to be absolute top draw and have a finer eye for detail in the paperwork.